The present disclosure relates to optical sensors, in particular to a method and apparatus for measuring a sensor comprising a plurality of optical resonators that are optically connected to a single optical output interface. The disclosure also relates to calculating a sensor signal based on response spectra for one or more optical resonators.
A photonic integrated circuit (IC) also known as an integrated optical circuit is a device that integrates photonic functions. Light traversing the circuit can be guided and/or processed by the circuit. One example of a photonic integrated circuit comprises a ring resonator arranged in proximity to a waveguide. Light in the waveguide may couple to the ring, e.g. by an evanescent field. The ring may have one or more resonant wavelengths that are selectively or predominantly absorbed or transmitted through the optical circuit as a result of interaction with the ring resonator. A resonant wavelength of the ring can depend on a physical (internal or external) parameter influencing the ring. Accordingly, the ring can exhibit a spectral signature that can be used to quantify said physical parameter, e.g. temperature, pressure, chemical data, et cetera.
The circuit can be interrogated (read out) by an apparatus, e.g. analyser. For example, the analyser can couple light into an input interface and read out light from an output interface to measure the spectral signature of the optical sensing element and quantify the physical parameter. The analyser can be a separate instrument for interrogating a photonic circuit integrated on a sensor chip. In this way different circuits can be interrogated by a single instrument. To provide a flexible connection between the analyser and circuit, light can be free-space coupled into the circuit via an optical port or interface, e.g. a vertical grating coupler. For example, the analyser comprises an objective lens, or an optical fibre that is brought in close proximity to the optical port.
An optical circuit may comprise a plurality of optical resonators, which may be configured to measure the same or different physical parameters. For example a biosensor array can be provided to measure a reaction process. The optical circuit may be designed such that each optical resonator has a separate output interface to distinguish signals from each of the resonators, e.g. as described by M. Iqbal et al. (“Label-Free Biosensor Arrays Based on Silicon Ring Resonators and High-Speed Optical Scanning Instrumentation”, IEEE Journal Of Selected Topics In Quantum Electronics, Vol. 16, No. 3, May/June 2010). The prior art discloses using a separate output coupler for each resonator.
To simplify coupling between the analyser and the optical circuit it is desired to provide a minimum of optical interfaces. However, when multiple optical resonators are connected to a single output interface, the spectral responses may overlap making it difficult to distinguish the signals of each resonator. This is especially difficult if the individual characteristics of the optical resonators are not a priori known e.g. due to variable manufacturing conditions and/or the initial response to the physical parameters to be measured. It is thus desired to provide a method and apparatus that simplifies measuring a combined spectrum of multiple optical resonators and isolating the sensor signal of an individual optical resonator having a priori unknown response characteristics.